Data compression is required in data handling processes, where too much data is present for practical applications using the data. Commonly, compression is used in communication links, to reduce the transmission time or required bandwidth. Similarly, compression is preferred in image storage systems, including digital printers and copiers, where "pages" of a document to be printed are stored temporarily in precollation memory. The amount of media space on which the image data is stored can be substantially reduced with compression. Generally speaking, scanned images, i.e., electronic representations of hard copy documents, are often large, and thus make desirable candidates for compression.
Generally, an image does not change very much on a pixel to pixel basis and therefore has what is known as "natural spatial correlation". In natural scenes, correlation is generalized, but not exact. Noise makes each pixel somewhat different from its neighbors, but the natural spatial correlation enables not only the compression of digital image data, but the performance of certain image processing operations on the compressed data. Contrariwise, in synthetic graphics and computer-generated text, the correlation tends to be much higher, with colors limited to a small set. The limited set of colors and high correlation make these image types good candidates for lossless compression, at possibly higher compression ratios than those achievable with lossy compression schemes.
Heretofore, a number of patents and publications have disclosed image compression and color correction, the relevant portions of some may be briefly summarized as follows:
U.S. Pat. No. 5,408,425 to H. S. Hou, issued Apr. 18, 1995, hereby incorporated by reference for its teachings, discloses a processor and method of computation for performing a discrete cosine transform.
U.S. Pat. No. 5,319,724 to S. M. Blonstein et al., issued Jun. 7, 1994, teaches an apparatus and corresponding method for compressing still images while remaining compatible with a JPEG transformation.
U.S. Pat. No. 4,275,413 to Sakamoto et al. discloses a color space transformation where information is placed into lookup tables and stored in a memory--where the lookup table relates input color space to output color space. Sakamoto teaches a "unit cube interpolation unit" having known vertices. The lookup table is commonly a three dimensional table since color is typically defined with three variables.
U.S. Pat. No. 5,581,376 to Harrington teaches the conversion of input device signals Rs, Gs, Bs, generated by an image input terminal, to colorimetric values Rc, Gc, Bc, the colorimetric values being processed to generate address entries into a lookup table to convert them to Cx,Mx,Yx,Kx colorant signals or any multi-dimensional output color space, which includes but is not limited to CMYK or spectral data. Values not directly mapped may be determined using tetrahedral interpolation over a hexagonal lattice where the lattice is formed by offsetting every other row in at least one dimension.